Woofer Pods

(Fig 1.1)

I guess the first questions will be - why did I choose this speaker - The brand and the size? I don't get too bent around arguing why one product is better then another. These Rockford Fosgate Punch Stage-2 speakers seemed to be a good comprimise between quality and price. My personal 'expertise' consists of Google'ing and reading so I try to make the best decision I can with the information I have and then consider that 'well enough'. So, with that said:

Why use a pair of 8" woofers? : Some people have used one or a pair of 10", 12" or larger woofers. It seems to me that, personally, giving up a good portion of the trunk area just wasn't something that I wanted to do. I noticed that the area in front of the strut on either side of the opening isn't hardly ever used. The question then became - could I use that area. A quick and dirty measurement convinced me that I could put in a pair of 8" woofers. I read that 10" subs typically have a lower Fs, a higher SPL or sensitivity rating, greater Xmax and higher BL rating compared to 8" subs. All things which add up to more sound for the watt. I guess that ultimately, it depends on what you want.

This space, squeezed in between the luggage finisher back and the strut bar is for me, normally wasted space. If I could put the woofer back there, I would not be losing much. The other thought was that if I could match the texture and color I could make it look like it was factory built. Stealth in plain view - as it were.

Oh .. yeah. Usual "Save Harbor Statement" - car is dirty .. blah blah.

The pocket is hard to get exact measurement of the ft³ but we should easily at least .3 ft³ You have the remaining width of the car between the pockets to add any remaining needed space. Holding the speaker in the pocket quickly confirmed that we have enough space to physically mount it here.

8" Punch Stage 2, SVC subwoofer with parabolic injection molded Polypropylene cone, 2" aluminum voice coil former, insulated 16 AWG tinsel leads, linear spider, 2" excursion, 30 oz motor structure, Cold rolled vented steel basket with removable motor cover, 8 AWG compression terminals, optimized for small sealed enclosures, 200 watts RMS/400 watts peak. Mounting depth 4.0".

"Bottom line: Fiberglass enclosure thickness is based on many factors and no one can tell you how many layers or how thick your subwoofer box should be because all enclosures are unique. ..... My rule of thumb for thickening subwoofer enclosures is a real rule of thumb. After the enclosure has cured I test it by pushing on various areas of the enclosure with my thumb. If I find a spot that flexes I know it needs more thickening."

Good enough for me. I will try to get it at least 3/8" thick and then use the "Rule of Thumb" as needed.
Note: I orignally was going for 1/2" thick but found that by the time I got to about 3/8" I was well and ready to call it 'good nuff'.

(fig 3.1)

The recommended sealed enclosure size is .25ft³ but the range in ft³ is from .15 to .35 - according to Rockford Fosgate - (Fig 3.1). There is also a 4.0" mounting depth requirement.

The box volume will directly impact the performance of the speaker. Larger enclosures will provide flatter response and deeper bass where smaller boxes will provide a bump in the response curve and generally higher output for greater SPL.

Note: If you wanted to go with the 10" speakers - P210S8 the recommended sealed enclosure size jumps to .65ft³ with the range in ft&sup3 from .50 to 1.00 and the mounting depth to 5.625".

I'm happy to go with the recommended enclosure size and will try for something close to that.

Let's get to some math. Fig 1 shows the recommended enclosure has dimensions of 9.5"x14.5"x6.25". Multiplying those together we come up with 861 in³. Divide that by 1728 and you get .49 ft³. I was puzzled for a bit until I realized they were measuring to the outside of the box. Earlier they suggested using ¾" MDF so next I took 1½" off each dimension. That gives 8"x13"x4¾" which then gives 494 in³ or .29 ft³. Finally, you have to subtract the speaker displacement. The P28S8 has a displacement of 0.024 ft³ we end up with 0.26 ft³ - close enough to the recommended 0.25 ft³ (we had a range of 0.13-0.35 ~).

Therefore, to find the volume we need before subtracting the speaker displacement, we get that speaker displacement in liters which is 0.610L (Fig 3). Now, add that back to the 7.08L and we get 7.69L ~

By the way, going back to our range for box volume of 0.15-0.35 .. adding back that 0.024 speaker displacement, the range is 0.174-0.374 ~

(Fig 3.3) Interior Volume

(Fig 3.4) Exterior Volume

Interior Volume : (Fig 3.3) Let's start by visualizing the required space. I am going to take a box that is 1ft x 1ft and however high it takes to contain my volume. Since the volume is a constant (for this math), then the surface area is also a constant. Since my volume is .29 ft³ .. that means a box 1.0'x1.0'x0.29' (about 3½" high).

Exterior Volume : (Fig 3.4) Now, let's add 3/8" to all the sides (this is the thickness of my fiberglass box and the volume is caculated off that. If you use a different thickness simply adjust for that). Our box is now 1.0625'x1.0625'x0.3525'. Multiply that out and you get a volume of .40 ft³. 1 cubic foot = 28.3168466 liters - so this gives us 11.33L ~ and THAT, is the final volume that we have to make room for.

(Fig 4.1) Sta-Green Perlite

(Fig 4.2)

Now, having got our volume we need, we will create an object with the desired volume that can be molded into the pocket. Once I'm happy with it, I can mark just where I need to bring the fiberglass up to. I picked up two bags of Perlite at Lowes (Fig 4.1). Each bag is 8.8L so two are enough to use for even the larger volume in the range.

(Fig 4.2) Perlite is a naturally occurring silicous rock. The distinguishing feature which sets perlite apart from other volcanic glasses is that when heated to a suitable point in its softening range, it expands from four to twenty times its original volume. When quickly heated to above 1600°F (871°C), the crude rock pops in a manner similar to popcorn as the combined water vaporizes and creates countless tiny bubbles which account for the amazing light weight and other exceptional physical properties of expanded perlite.

(Fig 4.3) 12+ Liters of Perlite in bag filled via measuring cup

(Fig 4.4) One bag of perlite ready to go

(Fig 4.3) Take a trash bag, probably a brand that is fairly strong as you will be poking, and shoving the bag. Perlite is light but still a rock and will easily tear plastic. A cloth bag of some thin material would probably be best. Fill it with .. in this case, 11.33 Liters of Perlite (I used a measuring cup).

(Fig 4.4) Fold up nicely, and secure with tape. Should be 'flexable' enough to mold to the interior of the car.

(Fig 4.5) Bag placed in pocket

(Fig 4.5) The bag is placed into the pocket and this is where it is good to have a 'bag' strong enough to survive all the poking, pushing, smushing, streaching and so on. The trash bag I used was a little too thin. While it worked ok in the end, I had to be very careful when trying to 'mold' it into the space because of the thinness of the bag.

Notice that the bag doesn't confirm to every 'nook and cranny'. I'm just trying to get 'good nuff'. Remember, we are trying for a final volume of .25 ft³ with a range from .15 ft³ to .35³ so I can easily say - 'good nuff!'.

Define the edges

Mark the edges of the bag I used masking tape to mark where the 'edges' of the bag lay against the interior. This will give me the point that the fiberglass must extend to. Later, when the MDF ring is positioned, I need to ensure that it is as if I set the ring on the bag.

The box then will be this size

Final box position marked Next, I will clean all this up .. make the lines nice and neat. I will then make a pattern so I can transfer the same markings to the opposite side.

Speaker positioned to fire at 45 degrees

Speaker positioned to fire horizontally

I tried a couple of different positions for the woofers to fire. I positioned the speaker at 45° - that will be firing pretty close to the rear hatch window - but with 8" woofers I wasn't really that concerned. If I was using something larger, like a 10" woofer then I would have at least a little more concern.

The second photo has the woofers firing straight to the right at 90° (suggested by Ninja350Z)- again, not sure if this would cause problems with them firing at each other - way beyond my audio knowledge of overlapping fields and all that ....

Woofer Template fitting - looking rearward

Woofer Template fitting - looking foward

I cut and taped poster board together, until I had a pattern of the sides and rear of where the box would go - this simply follows the masking tape outline that I created from the prior step. This will be used to indicate where the fiberglass will be layed up to (and past) and afterwards, where to cut since the pattern will allow you to use a permanent marker on the tape .. and that .. will transfer to the fiberglass. Once you have the pattern as you like,it can simply be flipped over, mirrored for the opposite side of the car.

(Fig 5.3) Woofer Template

(Fig 5.4) Driver's Side Template made pretty

(Fig 5.5) Driver's Side Template Mirrored

Finished Pattern : (Fig 5.3) Here, we have the pattern/template layed out flat. Not very pretty with all the cutting and taping but it is probably the best way to get everyting to fit right.

Pretty Patterns : (Figs 5.4, 5.5) In the two photos on the right is where I just cleaned up (photochopped) the template so it would look nice.

(Fig 5.6) Remove bolt and clip

(Fig 5.7) First layer of Masking Tape

(Fig 5.8) Second layer of Masking Tape

Remove 12mm bolt : (Fig 5.6) I removed the 12mm bolt holding a clip. I believe this is for a baby seat.

First Tape Layer : (Fig 5.7) I used 3M Masking tape to cover the area I will be fiberglassing plus a generous area around it. The first photo shows the masking tape with one layer. Each strip is overlapped.

Final Tape Layer : (Fig 5.8) The second photo shows the finished taping. I put a second layer of tape, again overlapping the tape below.

(Fig 5.9) Place template and mark around the pattern

(Fig 5.10) Finished pattern marked onto tape

Placing the Template : (Fig 5.9) The template is placed onto the tape and using a magic marker, I carefully traced around the pattern. For the opposite side of the car, the pattern is simple reversed giving you a mirror image.

Markings : (Fig 5.10) Tape has had the pattern traced onto it. When you are layering the fiberglass, the line will show through so you know where to lay the fiberglass to (actually a little past). In addition, the line will be transferred to the fiberglass showing where to cut the finished piece.

(Fig 6.1) Passenger side fiberglass shell

(Fig 6.2) Out of the car

(Fig 6.1) This is the shell with one layer of fiberglass cloth and two of mat. This being my first fiberglass project - I was feeling my way here. The fiberglass was about 1/8" thick I think .. and more matt would be added. I found a place selling boat supplies where I was able to pick up some heavier fiberglass mat there along with resin. I would like to have at minimum a ¼" thickness. I'm just going off - a feeling here. I'll feel it and decide if it's thick enough!.

Found out that using the masking tape to lay over has a couple of drawbacks. Not enough to not use it ... but (1) It leaves some residue. I'll have to pick up some 'Oops' to remove it and (2) It is REALLY hard to get the dang fiberglass part out!. That tape is now one freakin' big sticky joker! Like trying to pull a tooth the size of a basket ball.

(Fig 6.2) The shells have been pulled out of the car and are layed out. The one on the right still has the masking tape stuck to it. You peel that off and ... ummm ... now the fiberglass surface is sticky. When I was laying up the first layer of fiberglass cloth I used some spray adhesive to tack the cloth to the vertical surfaces. Suppose it will help when later in the project I stretch the cloth over it.

(Fig 6.3) Looking Inside

(Fig 6.4) Backside

(Fig 6.3) In this photo, the tape is mostly off and we are looking into the fiberglass shell. It is a lot easier adding layers of fiberglass to the inside with the shells out of the car. For one thing, will keep the smell out of the car. What's there isn't that bad .. but I would just as soon have it in the open air. It will also not have to be so careful with the resin.

(Fig 6.4) I flipped the fiberglass shells over for this shot. Not much to say. I need to start cutting the MDF rings out and get the additional fiberglass mat to increase the thickness of the shell.

(Fig 6.5) Finished Shell

Good-Nuff : Several layers later, some Bondo-Glass and now they are ready for the MDF rings. I ended up with about a 3/8" thickness. My "Rule of Thumb" indicated that should be "Good-Nuff".

(Fig 7.1) MDF Rings Completed

In the various articles on constructing MDF Rings, I have seen them constructed using routers with jigs, rotory saws used again with a circular attachment. Tech Wiki Tutorial on MDF Rings

I used a jig saw ... and can strongly advise you not to do so. It worked in the end but was a real pain. The jig saw does not cut straight, the blade bends so you end up spending extra time trying to fix that. I rounded over the edges of the rings with a sureform plane and sandpaper.

(Fig 7.2)

(Fig 7.3)

(Fig 7.4)

Test fitting I hot glued the rings into the woofer shell, making sure the ring was higher then the shell. I used scraps of wood to support the ring during the covering and fiberglassing. These temporary supports will be removed after the fiberglass sets up. The photos are of the is driver's side box, from three different views.

Note: I found out that if you leave the box in you car - in the sun - that the hot glue joint becomes REALLY temporary!

(Fig 7.5) Polyester Felt

(Fdig 7.6) Polyester with Resin

Covering : (Fig 7.5) Here I covered the boxes with polyester felt. Spray both the woofer shell and the edges of the polyester with spray glue (I used 3M brand). The polyester is stretchy so just keep pulling and pressing until you get an unwrinkled surface. The smoother you get the polyester the easier the finishing will be.

Coat with Resin : (Fig 7.6) Everything is looking good at this point. I put on a heavy coat of resin, soaking the polyester until the resin started to pool on the surface. I will allow it to dry overnight, and tomorrow I will cut out the opening and put a couple thick coats of resin on the inside, paying attention to where the polyester cover attaches to the shell.

Think I will purchase a Rotary Saw .. need something to cut out the top. I should have used one to cut the MDF rings. Ok .. next time ...

(Fig 7.7) Trimmed Excess Polyester

(Fig 7.8) Bondo and Primer

Trimmed off the Excess : (Fig 7.7) Removing all the excess material gave me a better idea of how this project was coming out. It also allowed me to test fit these jokers in the car. Yes .. I was getting antsy!!.

Bondo'd and Primer'd : (Fig 7.8) Sanded the surface, applied a layer of Bondo. Sanded this down with 60 grit sand paper. Applied a good coat of Glazing & Spot Putty, sanded with 100 grit sandpaper and finally sprayed a coat of primer. Since my intention was to apply a textured surface this was as smooth as I needed. If you want to go with a painted surface then wet sand down to 500 grit.

(Fig 8.1) Almost Finished

(Fig 8.2) Finished

Almost Finished : (Fig 8.1) Sprayed wrinkle finish, then sprayed a 'Vinyl Color' black tint. The speakers are just dropped in for this photo. I still need to drill the holes for the screws and find/modify something to cover the outside of the speaker mounting ring.

Finished : (Fig 8.2) Good nuff. I'm not completly happy with the wrinkle finish .. I need to practice to get it right. There were two cosmetic rings that came with the speakers. I cut ¼" off the outside edge, sanding them down and then used some Antique Copper Rub 'n Buff to color them.

(Fig 8.3) In Car

(Fig 8.4)

(Fig 8.5)

(Fig 8.6)

Installed Looks pretty good. The car is really dirty .. that will be taken care of - but it was 7pm when I finished and ... well ... it was beer time!